Unbound exciton-phonon states in oligothiophene crystals - A model approach for spectroscopic purposes

A simple model of exciton-phonon interaction in a crystal with one molecule in the unit cell is used to simulate some peculiar features of the oligothiophene absorption spectra. The model is treated in the limit of weak vibronic coupling, and the configuration interaction between the intense zero-phonon Frenkel state and the one-phonon continuum is described in terms of the Fano formalism. The good agreement between the calculated curve and a part of the experimental absorption profile allows one to discriminate some features that are physically relevant but are not accounted for in the model.